text:
What causes an airplane to rhythmically bounce during taxiing?
Some time ago, I was on a commercial flight and while the airplane was taxiing around, there was this rhythmic bouncing. Here is a video I made of the takeoff. You can notice the bouncing at the taxi and takeoff roll of the video. What causes that rhythmic bouncing? Is it a result of uneven wear on the tires?
For the sake of anyone who wants to know in case it helps, the airplane was a Boeing 757-200.

Not really obvious on the video. The nose gear running over embedded guiding lights maybe? In some places, it may be the junction between concrete slabs making up the taxiway, but this does not seem to be the case here.

Good question, I also experience it sometimes.

It can be caused by flat spot after tire locked after hard breaking. Once I flew on F50 where you can see tires through the window. And breaking flat spot was clearly visible.

If you are old enough to remember, the bias-ply automobile tires used in the 1960's would develop a flat spot when left sitting overnight. The flat spot would quickly go away when you started driving.
Many of today's aircraft tires still use bias-ply tires and are also subject to flat spotting.

Source: The Boeing 737 Technical Guide

Interesting, I have not been around for that long, so I would not know. The other thing is that the airplane was sitting at the gate for some 30 minutes. It had a previous flight too.

Interesting (I'd support it), but there are challenges: With the number of tires involved, and the fact that they will quickly be out of sync, due to the turns, can this effect survive a long time after the aircraft moves? Can we anticipate the frequency based on the diameter of the tires, and does it mach the video?

That is caused because the wheels are "uneven" after the plane was waiting at the stand. You have to imagine, that a plane has an enormous weight. The wheel are not fully pressurized like in a car, because they are going to expand in higher elevation.
Now, assuming an 44.5in (1.1m) tyre diameter (circumference ca 4m) and the taxying speed of approx. 17kts (ca 8m/s), we would hit the flat spot every ca 4 meters, so twice a second. That would induce oscillations. Im not a mechanical engineer, but i could imagine that this in combination with the dampers would cause the described vibrations. As the Boeing technical Guide is describing this as the cause I don't see any reason to doubt that.

So should we feel the same effect on every aircraft?

No, that depends on the tyre type (and therefore aircraft type) the turnaround time (how long the aircraft was parked) and other things like number of passengers and weight of cargo, as well as the temperature. It also depends on the speed of the taxiing aircraft. As a sidenote it should be mentioned that boeing started to use michelin as tyre supplier, same as airbus. So this should be a thing of the past.


text:
Why is there a maximum altitude to restart a jet engine?
On all the turbojets that I have flown the maximum altitude a pilot can attempt a restart is 30,000 FT MSL.  What factors go in to deciding what the maximum altitude is for an engine restart / relight?

I suppose if nothing else, unless they wanted to make it a rocket engine instead, they had to pick some value as the guarantee ceiling...

It's the minimum pressure needed to get the combustion going.

Basically, it gives the pilot the altitude and airspeed at which the engines can be relighted in flight. Based on these values, the pilot has to take a decision to descend etc. A jet engine requires (compressed) air, fuel and a flame source for (re)starting. So air density plays an important part in determining the maximum restart altitude.
In addition, there are other factors at play as well. In case of unassisted (without APU, for example) windmill restarting, the airspeed must be enough to overcome the inertia of the components. In certification memorandum on Turbine Engine relighting in flight, EASA says,

.. as core - bypass ratio of engines continues to increase,  higher  airspeeds  are  generally  required  to  enable  unassisted  windmill   restarting.  The  inertial  effects  because of the increased size, mass,  and number of engine driven gearbox accessories may also contribute  to   increased  rotor  drag  loads,  decrease  starting  performance,  and   possibly  contribute  to  rotor-lock  conditions.

In case a starter is available, the airspeeds required go down (as APU itself is usually a small turbine, the altitude effects are not as pronounced). According to CFR § 25.903 Engines:

(2) An altitude and airspeed envelope must be established for in-flight engine restarting, and each engine must have a restart capability within that envelope.
(3) For turbine engine powered airplanes, if the minimum windmilling speed of the engines, following the inflight shutdown of all engines, is insufficient to provide the necessary electrical power for engine ignition, a power source independent of the engine-driven electrical power generating
system must be provided to permit inflight engine ignition for restarting.

The engine restarting envelope is decided both in terms of altitude and airspeed. It also depends on aircraft. For example, in ETOPS aircraft, the APU must be designed to have airstart capability throughout the normal flight envelope. Regarding this envelope, EASA says,

2 Envelope of Altitude and Airspeed
2.1  Sufficient flight tests should be made over the range of conditions detailed in 2.2 and 2.3, to establish the envelope of altitude and airspeed for reliable engine restarts... The effect of engine deterioration in service should be taken into account.
2.2 Altitude and Configuration. From sea-level to the maximum declared  restarting altitude in all appropriate configurations likely to affect   restarting, including the emergency descent configuration.
2.3 Airspeed. From the minimum to the maximum declared airspeed at all altitudes up to the maximum declared engine restarting altitude. The airspeed range of the declared relight envelope should cover at least 30 kt.

Like most chemical processes, combustion becomes easier and faster with higher pressure. The combination of speed (which provides ram pressure) and altitude (which provides atmospheric pressure) must result in sufficient pressure in the combustion chamber. That is the reason for the stepped altitude limit. If density were the reason, there would not be a need to reduce the maximum restart altitude at lower speed.
For combustion to take place, first the fuel droplets injected into the air stream need to evaporate. This is made easier by higher temperature and pressure, and the lower both are, the more residence time of the fuel-air mixture in the combustor is required for good combustion. Next, the ignition system will add enough activation energy to start the combustion process. When the fuel-air mixture arrives at the ignitor, the fuel must have sufficiently mixed and evaporated for ignition to occur.
In case of the turbojets you have flown the pressure above 30.000 ft will not be sufficient to light the fuel, especially at the low compression ratio of the starter-driven or windmilling compressor.

@KorvinStarmast: Let's see: m dot is density times area times speed. For ignition, speed is bad: The longer the fuel has to evaporate, and the longer the fuel-air mixture stays around the ignitor, the better. You need to get the mixture ratio right, the activation energy high enough and the speed low enough, then ignition is ensured. Density is implicitly contained in pressure, because the gas laws tie them and temperature together. m dot is only important after ignition when a minimum thrust level needs to be reached.

Yeah, the memory was a bit sticky.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
What is the relationship between power and fuel efficiency?
We were having a discussion in our ground school that power will make the plane ascend or descend and that you use the stick for speed, forward for higher speed and back for slower speed. So does that mean as you fly higher, that means you increase power and ascend, and then fly from point A to point B, you will burn more fuel compared to if you went from point A to point B at reduced power, hence lower altitude?

It depends on a lot of factors, but in a word, no. Note that airlines go for fuel efficiency, and they fly almost as high as they are able.

possible duplicate of Airplane longitudinal control: pitch or power?

@Federico: that's certainly related, but does not seem to be duplicate as the main question here is efficiency.

@JanHudec you're right, I got confused by the introduction of the question (correctly addressed in Steve's answer)

raptortech97, that is exactly my point, but I am just unable to find a satisfactory explanation for the apparent ambiguity in my mind.

It's not this does that and nothing else. It is always a combination.
In climb you need to provide more energy to the aircraft because you are increasing it's potential energy. This can be taken from the energy which would have been spent to overcome drag in horizontal flight by flying slower. Less energy is needed to sustain the now lower flight speed, so some is left for climbing.
When you pull on the stick, you trim the aircraft for a lower flight speed (at least if you fly a naturally stable configuration), so some of the excess energy needs to be spent in other ways. The aircraft climbs.
If you advance the throttle, the trimmed speed will stay the same, but now more energy is available to be spent. It will be spent on climbing, because speed is already set by your elevator angle.
In a non-supercharged (normally aspirated) piston engine, the decreasing air density with increasing altitude will provide less air with every filling of the pistons. You need to lean the engine to keep the fuel-to-air ratio constant, so the engine will consume less fuel at the same engine speed, but also provide less power and thrust. Since air is also thinner for all other parts of the airplane, your true air speed will increase, your drag will decrease at the same true air speed and you can fly faster. But since your normally aspirated engine will provide less power, you need to advance the throttle in order to keep your attitude, speed, and climb rate. At some point, you will reach maximum power, and as you climb further, climb speed will decline until you reach the maximum flight altitude your airplane is capable of.
Flying higher will increase the efficiency of the engine due to lower air temperature, but this effect is small for piston-powered aircraft. Once you switch to supercharged aircraft, turboprops, and jets, however, their much higher maximum flight altitude will make flying higher markedly more efficient.
Flight speed, however, does make a substantial difference, especially for piston-powered aircraft. Their optimum cruise lift coefficient is $c_L = \sqrt{c_{D0}\cdot\pi\cdot\Lambda}$ and is rather high. Flying low means that you will fly a lot faster than what this optimum demands, and the higher you fly, the closer you will be to this optimum, simply because your engine will not allow a higher speed. That is why flying higher helps to fly more efficiently.

bonus points for trim = (hands-off) speed.  also, I don't know if this is a UK/US thing, but "unaspirated" is a linguistic term. In the US, weuse "normally aspirated".

@rbp In the UK, I think naturally aspirated is the common term.

That's strange, I'm in the US and I've always used "naturally aspirated".

Unfortunately, the cause and effect in this question is really muddled up. Peter Kampf's excellent answer should explain most of what you need, but it might be that you find his answer difficult to understand because you're still operating under the idea that the stick only manages speed and the throttle only manages altitude.
If this is the case, please consider this analogy: imagine that there are three buckets of energy available to any aircraft: chemical, potential, and kinetic.  (note for the pedantic: yes, chemical energy is potential energy.  Go away.)

Chemical energy is stored as fuel (dead dinosaurs).  It is released by the engine and can be converted into either altitude or airspeed or both.
Potential energy is altitude below you - the energy which is stored in the aircraft as altitude.  It can be converted into airspeed, but not back into dead dinosaurs.
Kinetic energy is airspeed - the momentum of the aircraft.  It can be converted into altitude, but not back into dead dinosaurs.

As long as the engines are operating, the chemical energy "bucket" is always pouring energy into the other two buckets.  The throttle determines how much energy is poured out per second.  The yoke (stick) determines the distribution of that energy - whether the kinetic bucket is pouring energy into the potential bucket, or vice versa, or neither.
What this means is that if you want to ascend, it's not enough to say that you simply have to pull back on the stick, or that you simply need to increase throttle.  Merely increasing throttle with the nose pointed at the ground does not help you ascend.  Merely pulling back on the stick with the airplane already at the critical angle of attack does not help you ascend.  To ascend, you need to "fill the altitude bucket", which (as Peter says) is always a combination of pitch and power.
As to your actual question regarding fuel efficiency, since it's based on false assumptions I will only say that it depends on the circumstances of the flight, but it is certainly not correct to say that increased fuel consumption is always the result of the greater throttle setting necessary to achieve a higher altitude.

You could pre-empt the pedantic by saying "gravitational potential energy"

After reading this my mind is fixated with the idea that there are dead dinosaurs stored inside my wings...

I'll add one thing about engine efficiency.
Aircraft engines are either more, or at least not less, efficient at higher power setting.

In spark-ignited (gasoline) engines the throttle adds resistance to the intake when closed, so they are most efficient with throttle fully open and slightly lean mixture so that all fuel is burnt. Modern directly-injected engines often support "ultra lean burn" when throttle is left open and power is regulated by injecting less fuel, i.e. very lean mixture, but most aircraft engines are older designs that don't work well with too lean mixture, so they are less efficient with less throttle.
Turbine engines are also more efficient at higher power. I am not sure about the reason, but probably because the high-pressure stage takes relatively less energy for driving the compressor leaving more power for the low pressure stage driving the propeller (at idle the power to propeller is minimal, but the core still spins quite fast; often around 60% rpm compared to full power).
Only diesel engines where power is controlled by amount of injected fuel only (corresponds to mixture in spark-ignited engines) are not less efficient at lower power settings, but at the cruise power they will not be any less efficient either.

And now combine with the fact, that your drag is about the same at the same indicated speed independent of altitude, but at higher altitude the same indicated speed corresponds to higher true (and therefore ground) speed.

...at least it does under the assumption that winds aloft don't vary with altitude. In practice they do, and so choosing the most efficient cruising altitude also involves looking at winds aloft.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How can you fly a plane without arms?
I hear about Jessica Cox; born without arms, she is the world's first licensed armless pilot.
How can you pilot a plane without arms? How are the controls modified in order to let her fly?
Anyway: great respect for this woman. Absolutely.

I read the title: I thought this was going to be about weapons

Jessica Cox explains it herself in this video:
https://www.youtube.com/watch?v=b2IqpPSF9-U
She's using her legs in place of hands (I.e, using her feet to manipulate the controls, throttle etc) and she specifically flies an Ercoupe which has no rudder pedals.


text:
What's with this non-exit Exit sign on Delta CRJs?
I have the pleasure of frequently flying Delta (i.e. Delta Connection) flights on CRJ-700 and CRJ-900 aircraft.  All of them have a bogus EXIT sign between First Class and the main cabin:

This is roughly half-way between the front doors and the wing exits.  This seems to me to make nonsense of any safety regulations requiring EXIT signage.

Maybe Delta's lately been carrying a lot of passengers capable of teleportation?

Might be on to something @Sean; it seems to me, like above, it's mostly vampires, as evidently they can't stand the sight of daylight.

Normally there is some kind of barrier, a bulkhead or curtain, separating the business class and the common folk in steerage.  In such a case the sign is to indicate that there is an exit somewhere down yonder beyond the bulkhead.
In this case the curtain is not there so the sign kind of hangs there with its nonsensical implication that maybe there is an exit right next to it.  I'll go out on a limb and guess that the regs require the sign to be there between the two classes regardless of whether a visual divider is in place.

I would guess that it's not required if there's no divider, but it's allowed, and not worth the effort to remove it.

Flew again yesterday (and I'm always in row 5)... the black strip at the bottom of the sign is a vestigial curtain hanger track.  So I guess we can convict on the evidence.

Sometime after 9/11, I noticed the walls between first and steerage were replaced with curtains, and then a few years later the curtains disappeared too.  My guess is that they were removed for "security" reasons, e.g. removing a chokepoint for terrorists could easily defend while they tried to breach the cockpit door.  But getting FAA approval to remove exit signs (a "safety" item, even if now unnecessary) on the brackets that held up said walls/curtains would be expensive, so the signs stayed.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Do any aircraft possess washed-in wing characteristics?
I have been teaching ground school at my local Canadian Air Cadets. When I was teaching about the wing twist of an aircraft, I was asked if any aircraft possess washin characteristics (vs washout). The book teaches that wing twist is either washin or washout. Washout is supposed to decrease the wingtip's angle of attack in order to promote better stall characteristics, while washin does the opposite (worse stall characteristics). Not sure why one would want washed-in wingtips, but was wondering if anyone knew of such aircraft.

According to Wikipedia, the Grumman X-29 had washed-in wings, as its considerable forward wing sweep gave it a strong tendency to stall at the roots.  As the wing roots on a forward-swept-winged aircraft are located aft of the wingtips, a root stall on such an aircraft would cause a strong pitch-up tendency, so the X-29's wings were washed in to mitigate the root-stall tendency.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why don't 747s use unreliable, but cheap, jet engines instead?
I'm a neophyte who was reading a little bit about jet engines, and the economics behind them. In particular, I was curious about what makes jet engines so expensive compared to piston engines.
A recurring theme that came up, again and again, was the cost of quality control, and related factors like product liability. Jet engines are designed to be ultra-reliable under conditions of high stress, and understandably that drastically adds to the cost of manufacture and design.
However, what doesn't make sense is why these standards are so universally high, even in the case of jets on multi-engine craft with redundancy. It would seem that you definitely want reliability on a single-engine craft, a multi-engine that can't fly when it loses an engine, or even a two engine craft.
But for a four-engine aircraft like a 747, why pay so much for ultra-reliability? A 747 can stay at cruising altitude with only 2 engines. Modern jet engines have a failure rate of about 0.01 per 1000 hours. A 747 could use engines with a failure rate of 1 per 1000 hours, and still achieve the same safety profile as an aircraft with only 1 engine of redundancy. (Assuming engine failure rates are independent)
I'd assume that relaxing the jet manufacturing process to allowing 100X less reliability would drastically reduce costs. It seems like we could be manufacturing very cheap four-engine aircraft by allowing for high-failure rate engines. What am I missing here?

Engines have heavy, rotating parts. If these crack, they cause a lot of damage. That can damage the plane, and kill people.  Read about this crash in Sioux city: https://en.m.wikipedia.org/wiki/United_Airlines_Flight_232. Or this very recent event: http://avherald.com/h?article=4b7725fb&opt=0 An other incident that was a very near miss was QF32: https://en.m.wikipedia.org/wiki/Qantas_Flight_32

How much more would you pay for a ticket on the more reliable aircraft? The cost savings would have to be enough that a competing airline using the most reliable engines they can get would not be able to take all your business with an ad that says something like, "Our engines are 100 times more reliable than some of our competitors" while showing a loving family with young children on their way to Disney World.

You wrote, "A 747 can stay at cruising altitude with only 2 engines." But what about engines failing during take-off on a hot day at maximum weight?  The desirability of jet engines is not just their fuel economy but their reliability; they are much more reliable than powerful piston engines.

Example: an airliner cannot depart due it cannot properly starts an engine, a nuissance can be quick fixed in 1 hour. That minor nuissance  will generate strees in a hundred passengers and they will remember that next time they plan to buy a ticket so sub par reliability is a liability to the airline image

I am very interested in where you got the information that a 747 can stay at cruising altitude with only 2 engines. This is definitely NOT true for 747-100/200 aircraft, and though I cannot from personal experience say that it is not true for -300, -400, and -800 variants, I would be very surprised if it was. Typically multi-engine jet aircraft try to operate at or near their optimum altitude, and an engine failure will make it impossible to maintain the optimum all-engines-working altitude.

Yes, I'm all set to go for a flight on that airline, You don't buy tickets, you buy chances.

I find it very hard to believe that you could save money like this. Failing a part in QA is orders of magnitude cheaper than having it fail during use. Or even during routine maintenance. "Fail early, fail often" is (I think) from software engineering but the economics does not change just because you are talking about jet engines. You have to remember you are not actually changing the fail rate (except possible making it worse) you are just moving fails later. And that makes them more expensive.

Think about it from a liability perspective.  If an engine failure resulted in a fatal accident and it was known that you chose a less-reliable engine to save money, you'd get the pants sued off of you and lose your business.  Even if the better engine wouldn't have prevented the accident, that's a PR nightmare you may never recover from.

@Penguin Please take the m. out of mobile addresses when you post links. Sites will redirect mobiles but not desktops to their appropriate versions.

TIL Neophyte. Thank you.

Legend has it that the chairman of Rolls-Royce would only cross the oceans if he could look out the window and see four Merlin engines.

"Lottery Airlines: we guarantee you take off. Everything else is TBD". You could sell funeral insurance along with the carbon offsets.

Watch some AgentJayZ, especially the parts about what materials are used and why they need to be in tight, expensive to manufacture tolerances - the consequence of using significantly cheaper materials or looser tolerance would not result in an unreliable engine, but in a reliably failing engine.... for example, using a cheaper alloy for turbine blades would cause them to just plain MELT when thermal equilibrium is reached....

The cost of a jet engine is mainly dictated by the operating conditions: high temperatures (sometimes above the melting point of the metals used) and high rotation speeds. This means you need high-performance materials even for a low-reliability engine: if you compromise material performance too much, it'll fail on the first start.
Failures are risky. Having to shut down an engine mid-flight is one thing, but we've had some recent failures where chunks of metal were launched at high speed into the wing and fuselage.
The industry has gone the other way: by making the engines more reliable, it's now possible to build a long-range airliner that uses only 2 engines instead of four, reducing the purchase, maintenance and fuel cost. It makes little sense to set up a special B-quality production line for a single aircraft type that's being phased out anyway.
Failures are costly. An in-flight failure means taking the aircraft out of service to repair the failure, which means having extra aircraft on standby to replace it.

An in-flight failure that merely results in the aircraft being taken out of service for repairs is your best-case scenario. "Substandard plane crashes over Atlantic, killing 100 passengers" is the kind of news article that could very easily kill an entire airline...

Ask the good folks on Southwest 1380 how important reliability is. Sure, most of them survived, but how many people would fly an airline where you're lucky to survive every flight?

Passed out sounds rash? There not selling well currently, but the B747 and A380 likely stay very long with us. Both jets work well and the freighter model of B747 is demanded, old models are re-used. Sadly the average-cost and no-hub concept are working against them.

Your assumption that failure rates of identical engines in identical service are independent is probably not true.  If it was true it would take a lot of proving, and the cost of that would end up adding to the costs of the engines.  Failure is not random but depends on age/wear -- the classic bathtub curve demonstrates that.  Then the load on the other engines increases if one or two fail, increasing the probability of additional failures.
On top of that think of the bad press for the airline which keeps having engine failures.  Even if the passengers on that flight didn't notice, the ones waiting for it to be fixed before they could take off would.  The knock-on effects on airline and airport operations of even a minor fault can get very expensive very fast too.

Also note a minor fault causing a delay in departure can cause lots of expenses on airport taxes and (almost ever) will cause a domino effect because that airplane is expected to keep going from hub to hub on a tight scheduled time

It's worth noting this has happened in real life: bad maintenance caused all 3 engines to fail during the course of the flight. One of the flight crew even radioed: "We believe it to be faulty indications since the chance of all three engines having zero oil pressure and zero quantity is almost nil."

@TemporalWolf: "O-ring" reminds me of... Challenger??

@Mehrdad https://space.stackexchange.com/a/17322/415

@MichaelKjörling: Wow, thank you for the link!

@TemporalWolf that had less to do with low reliability engines and more to do with "why there are ETOPS rules".

@Harper but even in reliable engines failures can't be assumed to be uncorrelated, which is what TemporalWolf's comment demonstrates

"What am I missing here?" is a good question to ask.  Cost of manufacturing is not the only cost involved, as other answers mention.  Other costs include:

How does the engine fail?  Does it simply drop in performance or stop operating, or does it throw parts or blow up, damaging the aircraft?  This may result in requirements for additional engines, kinetic shielding, etc. with impacts on payload, fuel consumption, etc.
Regulatory risk.  The FAA (or other national and/or local regulatory authorities) may, to protect passengers, aircrew and the people and property that airliners fly over:

Deny type certification
Deny permission to operate these aircraft
Restrict the allowed usage:

Routes, e.g. over-land only, not allowed over major cities (think 9/11)
Cargo-only vs. passengers

Business risks (reputation, lawsuits, marketing problems after a failure)
Advertising costs (to persuade passengers to use an airline after a string of engine failures)
Difficulties in recruiting good mechanics and aircrew
Increased maintenance
Passenger risk.  IIRC when a jet engine breaks free of a mounting pylon, it will tend to accelerate hard in a curved path and may 'shoot down' the aircraft it came from
Liability due to parts of the engines falling on people

In relation to the point about being restricted on certain routes, you could mention that certain A320neo's with PW1000G engines have had their ETOPS certification reduced because of poor engine reliability. Instead of being allowed to fly 120 or 140 minutes from a diversion airfield, the route cannot be more than 60 minutes away. That could costs airlines money, if they need to lease other aircraft, or swap engines, meaning more maintenance costs, and lost revenue with aircraft on the ground, not flying. See: http://avherald.com/h?comment=4b4c4509&opt=0

Regarding your point on passenger risk and engines breaking free, you might want to compare El Al Flight 1862.

Often engines do not breakaway at the intended shear points, and tear away other parts critical to flight, like the slat hydraulics. When the flight crew has a heaping serving of "deal with broken engine and changed flight behavior" on their plate, they simply don't have time/ bandwidth to deal with unforeseen side effects.

You are missing downtime and unpredictability
An air liner typically spends about 16 hours of every 24 hours per day in the air... for its entire lifetime. Every minute that an airliner spends on the ground, it is incurring cost, not revenue.
An engine failure means you need to pull the airliner out of service, get a replacement to take its slot, and put the failed plane in the shop to put in a new engine. Not only does this mean your regular plane goes out of service, you also need to have replacement planes standing by to take their place.
This — by the way — is why energy from renewable sources are not becoming any cheaper despite the hardware becoming cheap. The unreliable nature of wind and solar means you 1) need to have gas or other fossil energy available to jump in when the wind dies down, or you have poor solar availability 2) when everything is working fine you have too much energy and need to — in extreme cases — pay to be rid of it.
In the end it makes much more economic sense to have something that is slightly more expensive but reliable and predictable as opposed to something that is cheaper, but unreliable and random and that disrupts normal operation. Keeping things running smoothly is almost always preferable to being subjected to random interruptions.

On alternative power, that is being greatly mitigated by both energy storage and load dispatching, e.g. "Your tank water heater can put off its upkeep cycle for 5 minutes".  Hydro plants are retooling to get better at storing power, e.g. Oroville only had 3 reversible turbines but they are making all 6 reversible, they already have a massive lower pool for that purpose.  We'll figure it out.

@Harper That was word salad, and not of relevance to this post. "We'll figure it out". That promise has been made since the 1970's. for over 40 years people have said "Tomorrow renewables will deal with all our needs". That is not a bankable statement any more...

It's only word salad to people who don't know the field, i.e. who should not be talking about the field.

@Harper Get back to me after it has been figured out and is in operation. Words and hot air does not make energy.

Not without a turbine anyway.

Besides all the very good reasons in the other answers, one that has not been mentioned is that if you increase the failure rate by 100x, you increase the rate of replacing engines by 100x.
You can see in this example how an engine change takes several hours by several mechanics, with specialized tools.
Let's take your figures (I didn't check them) that a jet engine fails at around 100,000 hours. That's more than the usual lifespan of most airliners. Now you want to park your plane for, say, 8 hours, not once in 100,000, but once every 1,000 hours. On a regular lifespan of 100,000 hours, now engine changes account for 800 hours of downtime.  The cost to the airline for each day the aircraft is parked is enormous.
And then you have to account for all the aircraft shuffling every time a new engine fails. And you have to account for all the logistics to have 100x more engines available for replacement, and 100x times more crew. In the end, it's hard to think there will be any savings.

100x more crew is an overstatement.  With current engines, you still need crew, and they don't spend all of their time doing engine replacements.  Maybe you mean 100x more crew time spent on this task that (I think) currently accounts for only a small fraction of total maintenance/repair time.  The rest of the answer is good, though, working through the numbers that other answers only hinted at.  Good point that current engine lifetimes are more than the lifetime of an airliner, so this quantitative change in replacement rate could have a qualitative effect.

One way to look at current reliability metrics is ETOPS. Twin engine airliners are licensed to fly remote routes by meeting ETOPS standards. This dictate how many minutes away from a diverting airport they can fly, and it's based on the percentage likelihood they'll have an in-flight engine shut-down in 1,000 hours of flight. They need to achieve:
5% per 1000 engine hours for EDTO 120 minutes.
2% per 1000 engine hours for EDTO 180 minutes.
1% per 1000 engine hours for EDTO beyond 180 minutes.
Now most airliners don't meet these standards, and they aren't required for four engined planes (which I believe have different standards), but for the sake of this analysis lets assume 747 engines are at 5%.
A rough range of pricing for jet engines for commercial airliners is \$12M to \$35M, a 747 is obviously fitted with engines from the high end of the scale, so lets assume \$120M for a set of four. Assuming a 20 year life span, that's $6M a year in depreciation.
Now our new engine company (SuperCheapTurbines!, inc) shows up with our super duper low cost engines that provide the same performance as the \$30M engines for only \$1.5M each. The only trade off is they average a failure once every 1,000 hours instead of 20,000 hours but the airline will save $5.7M a year in depreciation. Note: I'm using 20x cheaper and a 20x higher fail rate for a more realistic comparison.
So how many do we sell, and do we decide to go public immediately or sell out to Boeing for billions? The answer is none, no, and no. Every airline declines our engines because paradoxically they say they are too costly!
Think about how that 747 is being used. Assume it's averaging 12 hours of flight time per day, and typically for a single long distance flight with an average of 300 passengers at \$500 each in revenue. So it's generating $150,000 per flight/day in revenue, or over \$50M a year. When it flew with the expensive engines, their failure rate meant it had an engine shutdown in flight roughly once every 400 days.
But with our engines fail about 10% of the time every 100 hours, and given there are four of them, the 747 would have a 35% chance of a shutdown engine every 100 hours. Here's the problem, while often airlines have the discretion to continue a flight with one engine out on a four engine airliner given the massive reliability of current engines, it's still a dangerous situation. If another engine goes out it will become a full fledged emergency, but for 5% engines a second engine failure would only be expected once every 1,000 - 2,000 single engine out occurrences.
But given how unreliable our engines are, there is virtually no chance regulators and airlines would permit flights to continue with only 3 engine operation, they would flag it as emergency requiring immediate abort of the flight. If one of our engines fails, there is roughly a 1 in 60 chance a second engine will fail on the same flight. So every engine out for a 747 equipped with SuperCheapTurbines! is likely an aborted flight.
And we'll have a lot of aborted flights. SuperCheapTurbines! equipped 747s will average an engine out once every 300 hours, or once every 25 days, costing over \$2M in lost revenue a year, and millions more in replacement engines. And that's just the start, every abort creates a huge amount of additional costs:

Repairing the engine.
Carrying more expensive backup 747s.
Lower fares because of the airlines terrible reliability.
And occasionally an engine out situation cascades into a triple engine out or other emergency, losing a plane with crew and passengers and costing the airline hundreds of millions of dollars in settlements and in  replacing the plane. And further in reputation.

Essentially this is why air travel has advanced so much from the propeller days. Back then to fly long distances safely designers added more engines, but the more engines they added the more likely an engine out situation became, or worse, an engine fire. Powerful, reliable jet engines have allowed airlines to switch almost entirely to twin jets that mathematically have far fewer engine out situations than three and four jet engine airliners, which means they can fly long distances safely even over water.
Note: I skipped over some things here, including normal maintenance costs which I assume are similar, and the higher interest costs for financing more expensive engines because I don't think they change much in the math.

There are several things wrong with this:

An unreliable engine will require more maintenance, which is really expensive and will almost certainly more than wipe out any savings over the life of the program.
A cheap, unreliable engine is probably also not as fuel-efficient, thus costing more fuel.
As Terry (a former 747 pilot) mentioned in the comments, your assumption that a 747 can maintain cruising altitude on 2 engines is not correct (or really anywhere close to correct.) With 2 engines, it would have to fly much lower, which means more drag which means more fuel burn and less range. If an engine is lost, the aircraft would still need to divert, which also means lots of extra cost to the airline and lots of mad passengers stuck somewhere in Canada until a replacement aircraft can be sent.
Advertising. "Our engines blow up 100x more often than our competitors'" is likely not an effective advertising slogan.

So, all told, this would be a false economy and would probably actually end up costing more in the long run.


text:
Are there any feasibility studies or research regarding the possibility of using Jet A in tropical countries?
Based on the ASTM D1655 standard specification for turbine fuels, the only difference between Jet A and Jet A-1 is the freezing point. Max [min?] -40 degrees Celsius (-40° F) for Jet A and max -47 degrees Celsius (-53° F) for Jet A-1.
Also, Jet A is widely available in the US (high latitudes).
From my understanding, if Jet A (with higher freezing point) is suitable in colder regions (e.g., the US), it should also be suitable in warmer regions (e.g., tropical countries), as it is already hard to reach -40 degrees Celsius (let alone -47 degrees Celsius) in tropical regions. Assume that both have the same altitude.
So, my questions are:
Are there any feasibility studies or research regarding usage of Jet A fuel in tropical countries? Please share!
and
Could someone provide information of the cost production differences between Jet A and Jet A-1?
Much appreciated guys!

What sort of "research regarding usage" are you looking for? Are you just trying to find out whether or not Jet-A is used in tropical countries, or are you looking for other information as well?

One thing is temperature on the ground, another temperature at the cruise level. Keep in mind it's worth climbing as long as it's getting colder.

And third thing is temperature at destination. And aircraft fuelling in a tropical country may be landing in an arctic one, or be overflying the North Pole, a couple of hours later. Do you want to complicate things by stocking multiple fuels?

@tanner-swett, for starter I want to know whether Jet A was used in tropical countries, and why don't they use it anymore. Like I said in my question, if Jet A is already used in colder region, it should be suitable in tropical region. So, why don't they use it? It is cheaper to produce (though I still don't know how much cheaper).

@jan-hudec, please explain "it's worth climbing as long as it's getting colder". Isn't it dangerous if your temperature at cruise level getting colder as it may cause the fuel freezing?

@JanHudec Yes, there might be some flights from tropical country to northern hemisphere, and for them you should use Jet A-1. However, my question is for domestic flight or international flight across tropical countries. And if it is worth the money, I don't see why we shouldn't stock multiple fuels. For that reason, I ask how much is price difference in producing Jet A and Jet A-1.

@arsenalist, the colder it gets, the more efficient the engines are. And note that the temperature at cruise level is almost always lower than the freezing point of the fuel. Average temperature at tropopause is -54°C. That's one of the reasons to use fuel-oil heat exchangers—minimum temperature for the aircraft is something like -67°C (IIRC that was for B777).


text:
Should all aircraft have landing gear debris deflectors, following the Concorde disaster?
Since it's certain that debris deflectors would have stopped the piece of tyre that punctured Concorde's fuel tanks in the fateful Air France 4590 disaster and caused an explosion, why are they not installed on most modern aircraft? They could prevent a similar incident in the event of a tyre explosion or other, even harder metallic debris that can be tossed/deflected onto the fuel tanks by the tyres spinning or the wheels.
Wouldn't this modification have saved the Concorde? Cost of maintenance aside, should it not be a requirement on all aircraft? Do they leave it off for aesthetic reasons?

Debris deflectors are there to stop debris that's been kicked up from the wheel. Stopping the parts of an exploded tyre would be a much harder job, so I dispute your claim that it's "certain that debris deflectors would have prevented the piece of tyre that punctured the concorde fuel tanks." Did the investigation report recommend that debris deflectors should be fitted to all aircraft? I doubt it.

@david-richerby If they are close to the wheel 6cm  to 9cm clearence, as in truck fuel tankers covering the top front and back of the wheel, they can stop just about anything especially parts of an exploded tyre depending on the material used its thickness density and strength. Lined with tyre like rubber material, they can certainly stop exploded tyre material.

Note that aircraft tyres are at much higher pressure than truck tyres (say, 200psi vs, er, well, I don't know exactly but Continental's sheet quotes load capacities for tyre pressures in the range 87-131psi). And weight is a big issue on aircraft: obviously, you could build something strong enough, but would it fly?

You can never really say "ignoring maintenance costs...", or "ignoring fuel costs...", or "ignoring passenger comfort..." etc. because those are exactly the factors that decide whether or not an 'improvement' is worth it. Every change is a balance between competing priorities, and weight/cost vs. safety/comfort is a 'classic' aviation dilemma.

"Do they leave it off for aesthetic reasons?" Please. Don't be ridiculous.

The tire debris did not puncture the tank; the debris caused a tank overpressure and rupture from the inside outward.

If you look at the current regulations for aircraft certification, at least half the rules can be traced back to a specific incidence. Why would aviation-grade hydraulic fluid be required to be non-flammable? Because of this Caravelle crash. And so on.
But at some point even the bureaucrats in the FAA realise that the benefits from adding the next gizmo will not outweigh the reduced performance and new failure cases of the modified airplane. In this specific case, it is better to require proper maintenance so nothing falls off an airplane in the first place, and frequent checks of the runway.

To illustrate your reasoning: "G-BOAB [another Concorde] during taxiing at London Heathrow. Burst of tyre No 2 leading to damage to the water deflector. Tank 1 suffered minor penetration, probably from a piece of the deflector". This is from F-BTSC accident BEA report.

Debris deflectors would not have worked for Concorde and certainly aren't universally viable, unless they were somehow designed to magically deflect all debris coming from all angles at all speeds away from any part of the airframe.
That's not going to happen.
What we have here is a freak accident happening due to a combination of freak coincidences.
Such a thing can never be prevented from happening. You could stay at home always so you can't get killed in a traffic accident, but you'd still be potentially killed by an asteroid hitting your home (yes, it's happened).
Same with aircraft and FOD. You can try to reduce the risk, but never completely eliminate it.
FOD patrols on airports are a good way, far better than technology that adds weight to the aircraft that works in some highly specific scenarios only.

I would hardly call the Concorde accident a freak accident; it was the result of a collection of errors of gross negligence and disregard for maintenance procedures and operating limitations. The debris on the runway was just the final catalyst that brought all elements together to lead to the flight's demise.

@JonathanWalters that debris being there was a freak accident.

But you describe a bunch of things as "freak coincidences" that @JonathanWalters refers to as "gross negligence and disregard". That's quite a difference.

@DavidRicherby "gross negligence and disregard" is just lawyer speak for "we want someone to blame so we can sue them for something that nobody could predict"

@jwenting Errrr, no. Gross negligence is lawyer speak for "you did your job so badly and dangerously that any idiot on the street could see that something catastrophic was going to happen." It's pretty much the exact opposite of what you claim it means.

Sections 4.2 and 4.3 of the Accident Report set out recommendations based on the investigators findings. Implementation of these recommendations would prevent such an accident.
The recommendations did not include landing-gear debris-deflectors.

Since it's certain that debris deflectors would have stopped the piece of tyre that punctured Concorde's fuel tanks

It might seem plausible but it is not certain.
See 1.6.2.4 Deflectors - a different type of deflector but an illustration that deflectors may also contribute to accidents - if the debris is large enough.
You can only be certain of the effectiveness of debris deflectors by actually testing with the relevant landing gear and identical debris at high speed. In particular you would need to test whether a debris deflector could catch a 43 cm curved metallic strip and the entire energy of a disintegrating tyre in the main landing gear rotating at very high speed.  In the absence of such testing, we cannot be certain.

and of course, if it can catch that specific thing, there's no way to know it won't make having something else on the runway (say a 30cm triangular piece) worse...

Putting arcane equipment on every single commercial aircraft to (hopefully) prevent a 1-in-a-billion accident is not a good idea.
Did you consider that when you add weird new equipment to a lot of aircraft that it can cause an accident? Did you know the FAA used to require every pilot to undergo real spin training... until they found out more pilots were getting killed in spin training than were getting killed in real spins.

What's "arcane" or "weird" about a debris deflector?

@DavidRicherby it is not a normal feature on an aircraft, or at least not on any aircraft I have ever flew, including commercial jets. The only place I have ever seen them (and only in pictures, not real life) is on military jets nose wheels that are flying out of dirt airstrips. I have never seen them on any aircraft on the main wheels ever.

There's a lot of room between "not a normal feature" and "arcane" or "weird". Boeing's unpaved strip kit for the 737 included nose wheel and main gear debris deflectors, as well as other modifications. You've probably never seen these things because you're not in an area where gravel runways are common (me either).

Since I cannot comment on any of the answers, I will just add an answer myself. There seems to be a misunderstanding of the accident. It wasn't a piece of tire that ruptured the fuel tank.
Also it wasn't negligence that caused the plane to crash.
It was a piece of aluminium that was attached as a repair measure on a boeing that happened to come off and ended up on the runway.
As far as negligence goes.... it was not detected, since the runway is NOT checked after every take-off, and the piece of aluminium happened to be from th plane taking off right before the concorde.
Just bad luck... :(

You could say it's negligence from that boeing repair team :)

while this addresses a misunderstanding behind the question, it does not answer the main question, that is valid despite the misunderstanding.

additionally, the misunderstanding was already addressed in the comments to the original question.


text:
What does "approved" mean in these quotes?

Class C service provides approved separation between IFR and VFR
aircraft, and sequencing of VFR aircraft to the primary airport.

Class B service provides approved separation of aircraft based on IFR, VFR,
and/or weight, and sequencing of VFR arrivals to the primary
airport(s).

What does approved mean?

What does the question in the first sentence have to do with the one in the last sentence? As noted in your comment on the accepted answer, you got and accepted an answer to one thing, but not the other. That's why 2 different questions in one post aren't allowed.

This is messy…. The original question was clearly and primarily about any differences between service in C vs B airspace.  Clarifying the term “approved” was an ancillary aid to a more complete understanding.  Seems we’ve dragged out a horse 6 years dead now for no good reason to define a term that was just a small part of the intent of the original question.  True, the OP shouldn’t have accepted the partial answer…

The term is not "approved", it is the complete phrase Approved Separation. This is the standard FAA term for what was previously called either standard separation or radar separation, and means "separation in accordance with the applicable minima in [FAA Order 7110]".
The separation minima are further defined in the JO 7110 family of FAA Air Traffic Control orders, which you can find linked from the page above.

What is the main difference between C and B then?

"Approved separation" means any separation specified in FAA Order JO 7110.65, Air Traffic Control, that has a minima (a numerical value) attached to it.  Like "6,000 feet and airborne" or "1,000 feet" for vertical separation, or "3 miles" for radar lateral separation.
What about visual separation, you ask?  Well, visual separation is a procedure by which separation may be reduced to anything below applicable published minima.

To answer both your questions in the context of faa-regulations:

"Approved separation" means separation in accordance with the applicable minima in this order. (JO 7110.65 1–2–1i)

These minima are prescribed because, according to the bureaucrats who created them, they are the best tradeoffs between safety and efficiency. (Separation standards are not prescribed in US Law [USC] or Agency-generated Regulations [CFR], but only in internal FAA Orders.)
This separation does not have to be a set distance. Planes may be separated by distance laterally and vertically, but they may also be separated by time and by other procedural practices (such as one-in-one-out at non-towered airports).
Separation between two IFR aircraft does not change based on airspace classification (though it can change based on the aircraft's altitude, and based on the type of radar system used by air traffic control).
"Approved separation" between two IFR aircraft is any of the following:

Three nautical miles laterally (or in certain cases five miles; sometimes more, depending on wake turbulence).
1000' vertically (or in certain cases 2000').
Pilot- or Tower-applied visual separation (not applicable in Class A).

These separation standards are the same in both Class B and Class C airspace.

The difference in separation standards in different airspace classes is related to VFR aircraft.
Class C Airspace, IFR-VFR

Target resolution (the dots on the radar screen don't touch).
500' vertically.
Pilot- or Tower-applied visual separation.

Class C Airspace, VFR-VFR
Traffic calls and safety alerts are provided, but there is no minimum separation.
Class B Airspace, VFR and "large" aircraft

One-and-a-half nautical miles laterally (sometimes more, depending on wake turbulence).
500' vertically.
Pilot- or Tower-applied visual separation.

Class B Airspace, VFR and "small" aircraft

Target resolution.
500' vertically.
Pilot- or Tower-applied visual separation.

I use "large" and "small" in quotation marks because they do not have their standard meanings (small: MTOW 12,500lb or less; small-plus: MTOW between 12,501lb and 41,000lb; large: MTOW between 40,001lb and 300,000lb; super: MTOW 300,001lb or greater). Instead, they are defined as:

"Large:" Any aircraft, helicopter, or rotorcraft, whether VFR or IFR, which either weighs more than 19,000lb or is a turbojet.
"Small:" Any non-turbojet aircraft weighing 19,000lb or less.

In Class D airspace no separation is provided between IFR-VFR nor VFR-VFR traffic, but traffic advisories and safety alerts are provided. In Class E airspace it is the same but, because VFR aircraft do not need permission to enter Class E, these advisories may not be fully complete.


text:
Why isn't airport relocation done gradually?
Recently Istanbul airport (IST) was relocated from Atatürk over 2 days. Munich Franz Josef Strauss (MUC) was also moved overnight from Munich Riem. Doesn't it cause chaos, since most employees are new to the place and equipment, work procedures are not well established? E.g. some people do not have badges with correct security clearance.
Why not do it gradually over longer time? Move airline by airline -- smaller first, bigger later.

Comments are not for extended discussion; this conversation has been moved to chat.

Moving airline by airline doesn't help that much:

You still have the same chaos, just on a per-airline basis.
The airports you mention are dominated by large carriers that have turned them into hubs (Turkish Airlines and Lufthansa). Even if you move all the other airlines one by one, you still have much of the pain of the big move when you move the largest airline.

And it has disadvantages:

Connections: A large percentage of traffic through these hub airports are connecting passengers, and thanks to airline alliances and partnerships, many are connecting between flights from different airlines. Very few passengers (and even fewer high-paying business travelers) will willingly break their journey to go for a drive across a famously traffic-congested city to change airports. Customers will abandon your airport and fly other routes while this is going on.
Equipment: When Denver International Airport moved, there was a massive overnight caravan "of more than 10,000 baggage carts, plane tugs, fire engines, catering trucks, de-icing machines and untold truckloads of tickets, tags and gift shop sundries" to the new airport. A similar operation occurred in Istanbul. If both airports must operate simultaneously, a fleet of equipment must be maintained at both airports during the overlap period. Much of this equipment is expensive, long-lasting, and will be difficult to sell or dispose of after the old airport is closed down.
Staff: There's not an exact linear relationship of airport staffing to the number of flights. Many staff may work for contracted ground handling companies and serve flights from more than one airline. They can't be in two places at once.

This is still done to a limited extent though. Turkish Airlines operated a few flights out of New Istanbul Airport for several months prior to the big move, which allowed them to test systems and familiarize staff with the new airport. Some of these disadvantages can be mitigated by limiting the number of flights and choosing them strategically.

+1 for the connections point. Passengers don't like having to make their way to the other side of the city because their inbound flight went to one airport, but their outbound flight moved to another.

I'm surprised that gift shop contents were part of the overnight shift in Denver.  I'd've expected that stores could be largely filled in advance at the new locations and then have their remaining inventory sent to the new location's stock room after the main rush.

Zach, that's fasicnating about the "overnight caravan" - good one!

Gradual relocation essentially mean having to staff and equip nearly two full airports during the transition period. It is also annoying for the travelers that want to transfer planes and need to relocate to the other airport. They would then need to get transported to or from the new location and through security again unless a small short hop flight is established during the transition. In IST's case it's 35 km distance between the old and new location.
Having two busy airports close to each other is also a bigger challenge for air traffic control than a really busy one and a calm one.
The solution to the issues you mention can be solved by thorough preparation. Like making sure all the old badges work (or having the new ones passed out as they come in for their first day at the new location), make sure everyone knows where they need to report for work in the new location. Perhaps having some extra trouble shooters on hand to fix teething issues.

Incidentally, the distance between the old and new Munich airports is also ~35 km (by road).

Gradually moving between airports is a living nightmare for connecting travelers.
Exactly that was done at Montreal Mirabel airport, a fabulous, spacious new replacement airport for Montreal Dorval (Trudeau).  Montreal used to be Canada's main international hub.  International flights were banned from the old airport, as incentive for airlines to move all operations to Mirabel. But they lacked the political strength to fully close the old airport, and never finished the high-speed-rail connection (or even highways) to Mirabel.  Passengers needed to take an hourlong bus ride and re-clear security.  This was so irksome that instead of consolidating at Mirabel, operators simply sent their international flights to Toronto instead, making it Canada's main hub.
They lost so many flights that Montreal didn't need two airports anymore, and they consolidated back at Trudeau. Mirabel's main terminal was scrapped and it's a race track now.  A few cargo operations remain.

Then you have the case of Kai Tak, where they "threw the switch" properly, but due to teething pains, threw the cargo operations back to Kai Tak for a short while.
Then there is Berlin.

On the other hand, some large cities get along just fine with 2 or even 3 major airports with one being primary for long-haul flights and the other being mostly domestic and regional flights. Off the top of my head, NYC, London, Chicago, Shanghai, Tokyo, Paris, Dallas, Houston, Washington, D.C., and Bangkok all work that way. Granted, the NYC airports aren't exactly an example of efficiency, but that's because each of them lacks sufficient space to build more runways, not because of failing to combine operations.

@reirab Or Los Angeles, with 5.  Yeah, NYC seriously needs to do the Mirabel thing.

@reirab JKF is primarily international flights while Newark and LaGuardia are primarily domestic, but they're not split cleanly like Mirabel/Dorval were. I can fly IND->EWR->LHR, as opposed to IND->EWR, bus/train to JFK, then JFK->LHR. That would be a nightmare and nobody would use JFK if they could possibly avoid it.

@FreeMan I had JFK/LGA more in mind than EWR. EWR is more split from the other airports by alliance than by domestic vs. international. All of United's long-haul operations are at EWR and their hub is there. Delta and American have their hubs at LGA and JFK instead with LGA being entirely domestic/regional and all long-haul operations being at JFK. A lot of Star Alliance airlines fly to EWR in order to access UA's route network, while oneworld and SkyTeam airlines mostly just fly to JFK, where they can access American and Delta route networks from JFK or LGA.

@reirab I picked EWR over LGA simply because my search for LGA (I'd totally spaced the name) also showed me "Newark" and "EWR", and it would have required another search to find "LGA". Call me lazy. Either way, if JFK were exclusively international, while EWR & LGA were exclusively domestic, there wouldn't be nearly the number of flights into all three that there are now. The fact that they are more airline/alliance split instead of destination split is what makes all 3 the major hubs they are.

@reirab I'm not sure I would say that DFW is "primarily for long-haul flights." I don't have the figures in front of me, but I'm almost certain the vast majority of flights from there are domestic flights.

@Patrick Yes, you're right. Love is entirely domestic/regional, but DFW has a good mix of both. JFK is really the only airport in the U.S. that is geared mostly towards international flights (though it, too, still has a decent domestic network, mostly for connecting to/from the international flights.) Perhaps I worded my original comment poorly. Those were examples of cities that have both airports with large international hubs and also one or more large airports with mostly or only short-haul flights. Though Tokyo is allowing more and more long-haul at HND these days.

What I mean is that all 3 should move their ops to a new airport far from NYC and have a HSR connection to downtowns. In other words do what Mirabel failed to do. Then no reason for a large plane to be anywhere near NYC.

@reirab EWR is only "primarily domestic" if you're a US airline that isn't United. There's a huge amount of international traffic from United and foreign airlines.

@reirab London doesn't really fit your long-haul vs domestic/regional split, either. City, Luton, Stansted and whatever other airports one might consider as "London" (*glowers at Southend*) are domestic/regional, and Gatwick and Heathrow do everything. If you're connecting from a long-haul flight to a domestic/regional flight in London, you'd typically do that at either Gatwick or Heathrow and wouldn't need to transit to another airport.

@DavidRicherby Yes, that's exactly what I said about EWR in comments above. Agreed regarding London, too. Pretty much all of the rest of the examples I mentioned work that way, too. My point was just that there's no reason a large domestic/regional hub can't coexist with a large international hub in the same large city. In all cases where that works reasonably, the international hub also has a large domestic/regional route network for connections.

Not least, British Airways and Southwest fly into London City from the US.  Oh wait, this is the real world... Only British Airways.

@DavidRicherby: I don't see how Luton, Stansted, or Gatwick could seriously be "airports one might consider as 'London'".  None of those are anywhere near London!

@Vikki reminds me of the line from Lost in Austen where Amanda Price (from modern time) mentions being from Hammersmith, "in London", and Mr. Bennet (1813) is puzzled: "I'm familiar with a Hammersmith, but it isn't anywhere near London!"

My experience was when KUL moved from Subang (now SZB) to the new KL International Airport (KLIA).
The moving date was declared way in advance; I seem to remember the date was locked more than six months before, and a lot of airlines rescheduled their operations, especially the night-stopping aircraft. Obviously, Malaysia Airlines had to ferry a bunch of planes over, but it was a 10-minute hop and done in the early hours, so it was not much of an issue. Some of the ground equipment was ferried over earlier in the day (of the last day of Subang operations), but everything else was moved over once the last flight of the day was completed. I'm talking motorized stairs, K-loaders, belt-loaders, tractors, trolleys, dollies - the works. It was quite a convoy of flat loaders. Stuff that could be driven on public roads were given temporary permits, so you saw motorized steps and water/toilet trucks on the public highways!
The biggest change was moving from a host (MH) check-in environment to a homegrown common-use system which was integrated with the Baggage Handling System (BHS). The first day's baggage handling was chaos, with a lot of bags not making their flights. A lot of items can't be duplicated, not only in terms of equipment but also in manpower, and it is easier to make a clean cut and manage the problems for a 24-hour period than drawing the pain out over a period of weeks.

OK but only your last sentence actually answers the question.


text:
Wing dihedral in a biplane
Would a biplane be able to fly if the upper and lower wings were in opposite dihedral? Imagine a wing configuration similar to the fictional "X-wing" design from StarWars. What would be the theoretical pros and cons of using such an approach? Ignore the engines, fuselage, tail, etc

What happens if you take it to its logical extreme and make all wings at 45 degree angles?

Does this answer your question? What will happen if an X shape wings mounted on biplane?

Sort of but not exactly. Here are the two area of difference.

In my question we are ignoring the engines - which are a major issue in that answer
In my hypothetical the wings are apart at the fuselage (just like a biplane) not touching as in that answer (which they say causes a ton of drag)

Added an admittedly crude image to explain better.

The main issue with biplanes is the separation between the planes, in relation to the wing chord. They need to be well separated to avoid interference and hence inefficiency. Anhedral lower plus dihedral upper increases the separation towards the tips, making them more efficient. Another way of looking at it is that the centre sections are bunched close together, making them inefficient. But the tips already suffer greater inefficiencies due to air spilling round the ends, with the centre sections providing the main lift. So what you are actually doing is making the whole wing inefficient. This is why many biplanes raise the upper centre section well above the fuselage on cabane struts.
As far as dihedral goes, the two angles balance each other out. If you want a stable plane you will need to increase the dihedral over the anhedral.
At extreme angles, say 45 deg, the wing offers significant side area. This configuration is in fact common on high-speed air-to-air missiles, in conjunction with a cruciform tail, as the control surfaces can now generate side forces and this can greatly improve manoeuvrability.

Rockets use cruciform wings because they have no clear up and down - no need to roll makes course changes more immediate. Banked wings create side forces, too, but take a while to reach the proper bank angle.

Yes, with rockets it amounts to the same thing. Whether the cross is vertical/horizontal or 90 deg or 45 deg makes little aerodynamic difference, but angling the wings rather than the tail makes it easier to mount the rocket to a hardpoint.

Brilliant answer, Guy. Thanks for sharing.

There's a line of airplanes that have already done it, at least a bit.
The Rutan tandem wing Quickie designs are exactly that configuration, except that the wings are radically staggered because the forward one is the trimming/pitch control surface, and the dihedral/anhedral angles are shallower.  But looked at from straight ahead, there it is.

The anhedral of the front wing is mostly there because that's where the wheels go. Versions of the 2 seat Q2 that have a conventional tricycle gear, have a nearly straight lower wing because the most of the anhedral is "in the way" (tips near the ground become a problem now without wheels there).
The problem with the configuration is other than being able to put the gear at the wing tips in Rutan's application, there's no point to it.  That anhedral wings cancel out the dihedral wings, form a roll stability perspective, making the result more or less the same as making all the wings straight.  If you do want to put the wheels there, you have to make the spars do double duty as really long leaf springs (bending loads from landing are much harder on the spars than flight loads), and there are ground handling issues with the wheels so far apart.
So might as well avoid the hassles of fabricating bent spar beams and make them straight across in the first place, or at least with more normal dihedral angles. It's more or less just something that looked cool from a science fiction perspective.

Would you consider this a biplane? I guess maybe? My thoughts were strictly one wing above the other!

@skanga: The term I've usually seen used to describe the Quickie is "tandem wing".

@Fred Larson  See paragraph 2 "Rutan tandem wing Quickie designs".   Anyway, no it's not a biplane, it's a tandem wing.  The front surface is the stabilizing surface, so some describe it as a canard, but tandem wing is more correct I think..

@JohnK It is wrong to describe a tandem fore wing as a "stabilising surface". Fore and aft wings are both primary lifting surfaces, with stability achieved through their relative arrangement (typically the fore wing has a higher AoA, but it gets complicated). All else is misguided sophistry (cue same, no doubt...)

@Guy Inchbald  I get your  point.  Better to say "trimming surface" since that is where the pitch control resides?

@JohnK In a tandem the centre of lift is roughly midway between the two wings. Elevators and trimming surfaces with sufficient moment arm can be mounted fore or aft (or, in theory, both), depending on the details of the design.

There's no particular need, from a physics perspective, to have dihedral in an airplane. It's there for stability. If you roll to one side, you naturally start slipping to that side. In an aircraft that has a positive dihedral, this slip means the lower wing will have greater lift than the upper wing. This creates a rolling moment back towards level.
An anhedral (negative dihedral) wing would have the opposite effect, increasing roll rate away from level. Note that this isn't necessarily a bad thing: it's possible for planes (such as the An-225) to be too stable, and thus, too difficult to roll when you actually want to. Having anhedral wings fixes that issue.
But there is another effect. Both dihedral and anhedral slightly reduce lift. Since the lift vectors of each wing aren't parallel, a small portion of the lift the wings might have generated is lost. This translates into slightly higher stall speeds, and slightly more drag.
In short, any "good" that the two opposite sets of wings might do would cancel each other*, while the "bad" would add together. You could get such an airplane to fly, it would just be less efficient than it otherwise would have been.

*Of course, if they're not at equal angles, or one is bigger than the other, or they have different airfoil shapes, etc., then they won't perfectly cancel, and you'll still get some effect based on which one is "stronger".

So, if I understand you correctly you are saying that the aircraft would fly similarly to a biplane with the same wings being totally flat EXCEPT that it would have a higher stall speed, and slightly more drag.

Yeah, pretty much.

Back in the 1980s (when Star Wars was still fairly fresh) there were a couple different radio control model designs that had the "X-wing" layout -- a strutless biplane with approximately equal upper dihedral and lower anhedral.  One was a canard, the other was tailless (in order to look even more like an X-wing from the movie).
They flew fine, with the very neutral "goes where you point it" stability of a (pre-3D, pre-Turnaround) Pattern aerobatic airplane.  They couldn't fly knife edge, because they had little rudder authority and couldn't keep the nose up at 90 degree bank (and were sloppy in point rolls for the same reason), but in non-aerobatic flight, they handled about as well as a conventional mid-wing, zero-dihedral Pattern design of the day.

That's interesting. Thanks for sharing ...


text:
What's the difference between Rotation speed vs takeoff speed?
My aviation teacher told us to memorize some speed values among other things before our flight training:

$V_R$ = 40 kt
$V_S$ = 39 kt

According to wikipedia:

Vr:
The speed at which the pilot begins to apply control inputs to cause
the aircraft nose to pitch up, after which it will leave the ground.

But I'm somehow confused with the values, since the takeoff speed (according to what I was taught) should be 10-25% greater than Vs, in this case around 43 and 49 kt.
If Vr is the definition mentioned above, which should be takeoff speed? does it have a definition within V speeds? Isn't takeoff speed the speed at which the aircraft leaves the ground?
Thank you

Based on the speeds that you are talking about, it isn't a turbojet definition that you are looking for.  The definition for $V_R$ that you gave is for turbojets.
In small GA airplanes, I teach my students to rotate around $1.3V_{S0}$ which really means to slowly bring the nose up to the takeoff pitch attitude.  Under no circumstances do I want them to yank the airplane off the ground.  If done correctly, the airplane will naturally lift off when it is ready to.

You need to add some time for the rotation to gather enough angle of attack change so the aircraft can lift off. While the pilot pulls to lower the tail and rotates around the main gear, the aircraft continues to accelerate. Also, the 1.3 factor is only required to be reached once the aircraft climbs through 35 ft (or 50 ft, depending on the certification). What happens is that the pilot starts pulling right around stall speed, the aircraft starts to produce more downforce at the tail until the nose gear lifts off, then rotates until the angle of attack has grown to maybe 10° so the wings create enough lift for take-off. At this time, the aircraft will have accelerated to around 1.15 times stall speed and starts to climb, which requires still more pitch change. Now drag will increase by the amount of induced drag produced by the added lift so the rate of acceleration slows down. Still, until the 35 ft are reached the aircraft will accelerate further to 1.3 times stall speed at this point.
That is how the take-off distance is flown for certification to achieve the minimum distance. Depending on the rotation rate (typically 3° to 5° per second) and excess engine thrust, the optimum take-off might require to start the rotation process at less than stall speed. For added safety, it is advisable to stay a little longer on the ground if the available runway permits it.

Given a target nose up pitch of 13 degrees, the first 747 carrier I flew for taught raising the nose to 10 degrees until liftoff and then going to 13. Their thinking was that that protected against a Vr bug erroneously set too low or a pilot rotating too quickly.

@Terry: This is the best way to avoid a tail strike. Takeoff run will be longer, though. If the runway allows for it, your old carrier's teaching is sensible.


text:
How do wide-bodied commercial aircraft achieve pressure equivalent to 6000 feet altitude?
I read an LA Times article which states the following.

To reduce the stress on the traditional metal frame, airlines now pressurize the cabin to be closer to the pressure outside. When a plane is cruising at 36,000 feet above sea level, the atmosphere inside the cabin feels like its 8,000 feet above sea level. For passengers, that can cause shortness of breath and fatigue.
But with new fuselages made of stronger, more flexible carbon-reinforced plastic composites, the plane can withstand more stress, allowing airlines to increase the inside pressure to a more comfortable atmosphere that feels like 6,000 feet above sea level.

Now I'm assuming that these are the twin-aisle wide bodied aircraft we see from Airbus and Boeing. The article mentions something about fuselage but doesn't get into the details of it. Can somebody elaborate?

did the needful.

The killer in fuselage pressurisation is the cycling of the structural loads. Aluminium is very unforgiving of repeated load changes: While steel has a load limit which can be applied infinite times, no such limit exists for aluminium. This means an aluminium aircraft will eventually fail if it flies enough cycles. It also means the plane can be built lighter or operated for more cycles if the pressurisation loads are lower.

Fatigue curves of steel and aluminium (By Andrew Dressel at English Wikipedia, CC BY-SA 3.0, source)
The consequences of this nasty characteristic were illustrated in Aloha Airlines Flight 243 in 1988, when the upper part of the forward fuselage failed at the suction peak aft of the cockpit and more than 5 meters of fuselage skin were ripped away.

Aloha Airlines Flight 243 after landing (picture source)
A fuselage made from carbon fiber reinforced epoxy shows very little fatigue and is much more tolerant of repeated loading changes. Here, thermal and impact loads are the most critical issues. However, the bad historic experience with aluminium structures means that not only the engineers, but even more the certification authorities are very careful to clear any new material for high, repeated loads. As a consequence, the graphite fuselage structure of the Boeing 787 is much stronger than a regular fuselage made from aluminium and can be loaded to a higher pressure without much added risk.
Whether this means less headaches and constrictions for you is less clear. The added pressure does feel better, but the major improvement in creature comforts in the cabin of the most modern airliners results from  higher humidity in the cabin air.
Most single-engined aircraft have no pressurisation; only the high end types like the Piper Malibu or the TBM 700 do. On the other hand, some business jets can keep the cabin at sea level pressure up to 12000 m altitude. In airliners, however, holding the cabin pressure at or above the equivalent pressure at 8000 ft of altitude is mandatory. Citing Wikipedia:

Keeping the cabin altitude below 8,000 ft (2,400 m) generally prevents
  significant hypoxia, altitude sickness, decompression sickness, and
  barotrauma. Federal Aviation Administration (FAA)
  regulations in the U.S. mandate that under normal operating
  conditions, the cabin altitude may not exceed this limit at the
  maximum operating altitude of the aircraft.


text:
Do people get less airsick if cabin pressure is at 6000 ft.?
This is in relation with a question I asked yesterday . This is part two of the question. Do people feel less discomfort. Whenever I'm traveling on planes, at times I feel strain in back of the head, sometimes my ears get closed, sometimes the throat,nose either get fully dried or they start to blow. Would a lower cabin pressure help in any of those things ?

If you travelled on an equally-crowded bus or train at sea level, wouldn't you experience most of those things?

If by airsick you refer to the typical meaning which is synonymous with motion-sick, then no. Low level flight at hot, humid, turbulent altitudes in an unpressurized aircraft will contribute to airsickness much more readily than flight at cool, dry, less-turbulent altitudes. If you mean discomfort due to onset hypoxia or other pressure-related issues, then yes, any lower cabin pressure will be better than any higher cabin pressure. However, some of those issues are due to rate of change or total change in pressure in descent, not the cruise pressure.

Let's be careful when discussing cabin pressure in relation to altitude. Using the words 'higher' and 'lower' can be very confusing here - even without the first signs of hypoxia...

The cabin air is dry, on purpose. This is likely for a large part at the origin of your discomfort.

Boeing supported a study that directly addresses your question:

On the basis of our findings, we conclude that maintaining a cabin altitude of 6000 ft or lower on long-duration commercial flights will reduce the occurrence of discomfort among passengers.

Note, though, that the lower cabin altitude of the 787 is one of its selling points, and the study was conducted not too long before that type's introduction. However, its methods seem reasonable (at least to someone like me with no background in health sciences) and it was published in an important peer-reviewed medical journal.
The Aerospace Medical Association took a broad look at existing literature on the subject shortly after Boeing's study. That review

did not find sufficient scientific data to recommend a change in the cabin altitude of transport category aircraft;

however, the authors did not dismiss the notion that a lower altitude may increase comfort, and they recommended further study. Just this month, some results of that further study were released. The results showed

no adverse effects from an environment pressurized to the equivalent of 8000 ft compared to 6000 ft,

although they did not account rigorously for the subjective "discomfort" that I think you mean by "airsick."
My reading of the current research (based on the above) is that there is no evidence to suggest that a 6000-foot cabin altitude is medically better than a higher altitude (up to 8000 feet), but it might make some folks subjectively feel better.


text:
Why do the wing tips on the Boeing 777X fold up instead of down?
The outer 11.4 feet (3.8 meters) of the wings of the Boeing 777X are designed to fold up when on the ground. Why aren't they designed to fold down?
It seems to me that folding down would be more "fail safe" if the locking mechanism failed in the air. The wings are generating lift, so they would hold themselves up and continue to contribute to keeping the plane aloft.
Up-folding wings could lift themselves if the locking mechanism failed in flight and destabilize the plane. The folding bit is less than 12 feet while the diameter of the engine nacelle is greater than 14 feet, so it doesn't seem like proximity to the ground is the issue.
What are the biggest reasons for folding up instead of down?

Navy aircraft have folded their wings up for decades, so presumably they have the mechanisms pretty well figured out.

With folding-down wingtips, you're restricted (w.r.t. the length of the folding part) by the groud clearance. There is plenty of space in the upwards direction.

It's not that big a deal.  It's fairly easy to make a latching mechanism that is incapable of operating under load, a great example is extension ladders or an over-center dog mechanism on a ship door.
Even mechanisms which aren't actually intended for that, are very hard to release under load simply because they bind.  Try to release one of these under tension!

And this one's not even trying.  It would be easy to make a mechanism which maximizes this binding force.
In the case of wingtips, wings have significant force bending the joint upward in flight (lift), and significant force bending the joint downward in taxi (the weight of the wing). That makes it all the easier to design such a mechanism, that is passively unable to release in lift.
Simply design the latch so there is considerable binding force when in lift.  Size the actuators to be far too weak to operate it against that binding force.  And/or put an intentional weakness in the operating lever where it'll shear off well before force becomes large enough.

_ It's fairly easy to make a latching mechanism that is incapable of operating under load_ - yes, and it's actually quite a tough job to design a mechanism that can be released under load that is comparable in load capacity and reliability to those mechanisms which can't.


text:
Does the 787 gust alleviation system make a more comfortable ride or for a more stable aircraft?
The 787 active gust alleviation system, similar to the system used on the B-2 bomber, improves ride quality during turbulence.
Ride quality in a B2 bomber means a more stable weapons platform.
Ride quality in a 787 means passenger comfort.
(I must add that I have never flown in a 787.)
Is the 787 more comfortable than a normal plane of similar size?

Yes it would because if there wasn't this kind of system on a 787 turbulence would be extreme

Note that other airliners are also equipped with similar systems (e.g. LAF on the A380)

Both, but the improved comfort and stability are only side effects.
A gust alleviation system uses trailing edge flaps (ailerons, elevators and sometimes spoilers) to counteract local lift changes caused by flying through up- or downdrafts. The flap is deflected to change the local incidence and camber of the wing in order to reduce the change in lift due to gusts. This helps to lower the collective cyclic loads the structure has to endure over time and allows to use a lighter, less strong structure, or it allows to stretch out the inspection intervals for the same structure.
Since the structure will experience smaller deviations from the average load factor, you, the passenger, will experience a smoother ride in gusty air. In calm air the system will not make any difference.
Also pitch changes are smaller since the flight will be less bouncy and the tail surface lift can be smoothed out over time in the same way. Therefore, a gust alleviation system works similar to artificially enhanced stability.
A major problem is the unavoidable lag in reaction. If the bending moment in the wing spar is measured, a computer has to calculate the appropriate response, has to command changes in hydraulic pressure and flight control surfaces need be moved. By the time the flow around the wing is affected by the flaps, the gust has long passed. Therefore, it needs instruments like laser anemometers to measure the wind ahead of the airplane so the reaction can be timed correctly.
The answer to your last question would primarily be based on opinion, so I leave answering it to you.

A320-100 had a load alleviation (the same thing) and AFAICT it was deleted on the -200 and the other variants that are based on it.

I think this answer is perfect. I can add/remove nothing.

@JanHudec why? It didn't work?


text:
Why did Boeing choose to give the Boeing 720 such a designation?
The Boeing 720 was a modification of the Boeing 707. Why was it given the unusual number of "720" instead of a "707-xxx" or "7x7" designation?

Because the launch customer, United Airlines wanted it. From Boeing frontiers:

The 720 was a ... first marketed to the airlines as the model 707-020. United Airlines was very interested in the 707-020 but had previously decided to go with Douglas and the DC-8. To help United avoid any negative public relations for going back to the 707, Boeing changed the name of the 707-020 to the 720.

According to aviationweek, the name change was at the behest of William ‘Pat’ Patterson, the president of launch customer United Airlines.

You'd think it would be negative PR for Douglas, but these decisions don't need to be rational...

@OrangeDog I really doubt that a company cares about the PR results for another company that performed below expectations, which is why I assumed United moved away from Douglas.

@NateKerkhofs Exactly, it's not clear why Pat though that would be bad PR for United.

I would suppose they wanted to avoid the image of having bought sub-quality parts. If you found out  one of your suppliers your restaurant had an E-Coli outbreak, but you caught it in time to prevent it from getting to your shelves, you would still want to be discrete about switching suppliers even though it's their fault, not yours - your PR would still be effected. That's not to say necessarily that their was anything so drastically wrong with the DC-8, but it is an example where switching suppliers because there was a problem could be bad for PR.

On second thought, I doubt it was like that at all. I bet it was more analogous to a new iPhone launch. We don't think of new airplane launches quite the same now, they are mostly about efficiency and so forth. In the sixties, airplanes were advancing with speeds rivaling Moore's law. Imagine Apple launching an iPhone 4, having a lackluster performance or availability problems, and switching back to a slightly modified iPhone 3. Would they want to call it an iPhone 3? Would that be good for PR/sales?

@wedstrom continuing that digression, Intel's "Core" CPUs all trace their ancestry back to the Pentium Pro through Pentium 3 and not the Pentium 4, which turned out to be pretty much an evolutionary dead end. And yeah, in the process they re-branded and now "Pentium" is only used for low-end stuff.

@OrangeDog Some people just hate anything that could be interpreted as an admission that they were wrong.

The 720 first flew before the 727 project began. At that time, there was no "7x7 series" – just the 707.
(The 717 passenger plane didn't come along until the 1990s, though the internal Boeing model number of what are usually called the C-135 Stratolifter and KC-135 Stratotanker was also 717, and that did exist at the time the 720 was produced.)


text:
What is wrong with my understanding about total pressure for a rocket nozzle?
I’m using the ideal rocket equations and isentropic flow but cannot determine what is wrong in my understanding about total pressure. Can anyone explain where I am wrong using the following example?
Example:
$\gamma = 1.4 $
Converging-diverging nozzle Area ratio $A/A^\star = 12.0$
Inside the rocket chamber $P_0 = P_t = 12 MPa$
Exit area $A_e = 0.002 m^2$
Mass flow rate $\dot m = 2.5kg/s$
$Thrust=5000N$
Supersonic flow
Atmospheric pressure $P_{atm} = 101.3 kPa$

Solution:
Based on isentropic equations - (e.g. https://www.grc.nasa.gov/www/k-12/airplane/isentrop.html)
Mach No. $M = 4.127$
$P/P_t = 0.0056$
$T/T_t = 0.227$
Substituting in $P_t$ gives:
$P = P/P_t * P_t = 0.0056*12 MPa = 67200 Pa $
Then based on the ideal rocket equation (i.e. https://www.grc.nasa.gov/WWW/K-12/rocket/thrsteq.html)
$Thrust = v_e * \dot m + A_e (P-P_{atm}) $
Rearranging for exit velocity:
$v_e = \frac {Thrust - A_e (P-P_{atm})}{\dot m} $
$v_e = \frac {5000 - 0.002 (67200 - 101300)}{2.5} = 2027.28 m/s$
Then the exit density can be found by rearranging this (also from the isentropic equations website):
$a = \sqrt{\gamma \frac{P}{\rho}}$
Rearranges into:
$\rho = \frac{\gamma P}{a^2}$
Speed of sound
$a = v_e / M = 2027.28 / 4.127 = 491.22 m /s $
Substituting into the density equation:
$\rho = \frac{1.4 * 67200}{491.22 ^ 2} = 0.390 kg/m^3$

And here is the problem/confusion:
From the question the total pressure is $P_0 = P_t = 12 MPa$
This is also supposed to be the total pressure at the nozzle exit because it is reversible/isentropic flow.
But $Total Pressure = Static Pressure + Dynamic Pressure$
$Total Pressure = P + 0.5 \rho v_e^2 $
$Total Pressure = 67200 + 0.5 * 0.390 * 2027.28^2 = 868624 Pa$
And this is nowhere near 12 MPa... so what part am I not understanding correctly?

$\frac{1}{2} \rho V^2$ is valid for dynamic pressure at low subsonic speed. At sonic speeds and above, compressibility effects are significant and the equation is $\frac{1}{2}\gamma \cdot p \cdot M^2$.

Oh, right I didn't realise it had a different equation for compressibility... and I just looked at https://en.wikipedia.org/wiki/Impact_pressure to see why the $\frac{1}{2}\gamma p M^2$ still isn't the full value

I think that this is definitely on topic here but if you don’t get a suitable answer, you might also try Space.SE.  Great first question by the way.  Keep ‘em coming!

I'll just answer my own question thanks to Koyovis' comment  :)
Koyovis was right - I didn't realise the difference between how dynamic pressure is calculated for compressible vs incompressible flows.
There are some useful notes on it on pages 4-8 of this link (http://mae-nas.eng.usu.edu/MAE_5420_Web/section5/section.5.5.pdf)
In summary:
Bernoilli’s equation is
$Total Pressure = P_{static} + P_{dynamic}$
For an incompressible flow,
$P_{dynamic} = 0.5\rho v^2$
For a compressible flow, use the isentropic equation
$P_t / P_{static} = (1 + \frac{\gamma – 1}{2} M^2) ^ {\frac{\gamma}{\gamma-1}}$
To get this in a way you can compare to Bernoilli's equation, make total pressure the subject of the equation, add & subtract $P_{static}$ from the right hand side, and rearrange the equation slightly (shown in the link above) gives:
$P_t = P_{static}  +P_{static} ( (1 + \frac{\gamma – 1}{2} M^2) ^ {\frac{\gamma}{\gamma-1}}-1)$
This is the same form as Bernoilli’s equation where the dynamic pressure is:
$P_{dynamic}=P_{static} ( (1 + \frac{\gamma – 1}{2} M^2) ^ {\frac{\gamma}{\gamma-1}}-1)$
Putting in the values from the example gives the correct result.

I don't think Koyovis was right...
$$ q = \frac{\rho V^2}{2} = \frac{1}{2} \frac{P}{R T} \gamma R T M^2 =  \frac{\gamma P M^2}{2} $$
and you run the numbers and they are identical...  (AC engine Design, Mattingly, pg13)
The link from Utah State uni on slide 7 does give different numbers as you have said in your answer Waterdragon.
$$ q_c = p \left( \left[ 1 + \frac{\gamma - 1}{2} M^2 \right]^{\gamma / (\gamma - 1)} - 1 \right) $$
Happy to be proven wrong and I have to say this was new to me.

Nick, welcome to Aviation.SE. We use LaTeX formatting for equations. I edited your answer accordingly (if I messed something up, please edit again). You can look at how I did it in the revision history by clicking on side-by-side markdown.

Yeah that wasn't the full equation.


text:
Why didn't Cirrus make the SR-22's parachute steerable?
After the parachute is deployed there is no control where the plane is going, so why didn't Cirrus make the SR-22's parachute steerable?

I'm just guessing, but since you can create some pretty significant descent rates over-steering a chute this may be more dangerous than it is beneficial. On top of that, additional training would be required to effectively steer the aircraft under a chute which most passengers wouldn't take. A big selling point of the chute is for passengers to be able to deploy it if the pilot is incapacitated.

I guess if you'd like to own a Cirrus and survive in emergency case, you would get additional training if it was available and learn how to steer a parachute. I agree with you in case pilot is incapacitated, but it could have both modes.

Upset recovery training is also good for surviving emergencies, as well as IFR because "inadvertent flight into IMC" is a major cause of accidents for VFR pilots, yet a lot of people elect not to take those additional training. And other than destroying an airframe or developing an advanced simulator, its difficult to train for just reading about it on paper...

Remember what the purpose of the CAPS/BRS airframe parachute is: It's a last-resort emergency measure to put the aircraft on the ground in a condition that is likely to be survivable for those onboard.
Steerable parachutes add considerable complexity: Control lines would need to be routed into the cabin, and connected in such a way as to not interfere with the chute deployment under any circumstances.
Assuming a suitable design could be devised to accomplish that a steerable parachute (even for a single person) requires quite a bit of training and skill to use. This is not something that can be adequately trained for in the case of the Cirrus airframe chute (because pulling the chute on perfectly good aircraft to teach steering would be cost-prohibitive).
Faced with the risks and complexity of a steerable chute or the relative simplicity and safety of a simple drag chute to decelerate the airframe prior to impact (hoping you land in a suitable 40-by-40 foot square) Cirrus and BRS opted for the latter. The result is a dead-simple "Pull the handle!" system that's proven very successful thus far.

Not to mention, If gliding the airplane to a safe landing were an option, you wouldn't have pulled the chute.